orbicularis oculi

The orbicularis oculi is a facial muscle
that has three parts – an orbital part on and around
the margins of the orbital opening; a palpebral part in
the eyelids; and a lacrimal part related to the lacrimal
gland.

The orbital part is the thickest and widest part. It extends
laterally into the temple, downwards
into the cheek, overlapping the muscles of the upper lip, and upwards into
the forehead, where it blends with fibers of the frontal belly of the occipitofrontalis.
The fibers of the occipital part arise from the medial palpebral ligament
and the adjoining part of the medial orbital margin, sweep in curves round
the orbital margin and come back to the same point medially, having no attachment
laterally except a loose attachment with the skin. Some of the fibers that
arise from the bone above the palpebral ligament are short, and end in the
skin of the eyebrow.

The palpebral part is paler and thinner than the orbital
part. Its fibers also arise from the medial palpebral ligament and the adjoining
bone, and sweep round in curves, having no lateral attachment. At its periphery,
the palpebral part is continuous with the orbital part. At the margin of
each eyelid, a small bundle is partially isolated, and is called the ciliary
bundle because it lies close behind the roots of the eyelashes.

The lacrimal part is a small thin sheet that arises from
the posterior margin of the lacrimal fossa and the facial sheath of the
lacrimal sac, curves forwards round the lateral side of the lacrimal sac,
and divides into slips that run laterally in the eyelids.

Actions of orbicularis oculi

The firm connections of the muscle are at the medial corner of the eye;
therefore, when it contracts, it draws the skin and the eyelids medially,
and thus promotes the flow of the lacrimal fluid medially towards the lacrimal
canaliculi. The palpebral part can contract independently to close the eye
lightly, as in sleep and in blinking. The orbital part also can contract
independently, screwing up the skin around the eye to give it partial protection
from a bright light or wind and rain; and those of its fibers that are inserted
into the skin of the eyebrow draw the eyebrow down in frowning, and draw
the eyebrows closer together, producing one or more vertical furrows in
the middle of the forehead. Both parts contract firmly to protect the eye
from an unexpected bright light or sudden danger. They contract firmly also
during strong expiratory efforts – a spasm of coughing, hearty laughter,
the crying of a child. This is probably to compress the contents of the
orbit and prevent over-distension of the orbital veins. The lacrimal part
helps the palpebral part to draw the eyelids medially; by its connection
with the facial sheath of the lacrimal sac it probably dilates the sac and
widens the openings of the canaliculi.